The present invention relates to a field effect mode liquid crystal display device in which the retardation in its liquid crystal layer is compensated for to attain high contrast and good viewing angle characteristic, and an elliptical polarizer.
In order to realize a liquid crystal display device with high contrast, a display device is proposed in U.S. Pat. No. 4,844,569 in which two liquid crystal elements with their spiral structure twisted in the directions opposite to each other are stacked or birefringent plastic films are stacked. The birefringent plastic films are bonded to be stacked by bonding agent between a polarizer and an electrode substrate.
Previously known liquid crystal display devices capable of displaying a large capacity are those of a super-twisted nematic (STN) type and a thin film transistor (TFT) type. The STN liquid crystal display device which is characterized by a larger twist angle in a spiral liquid crystal layer has an excellent multiplexed driving characteristic.
However, in these liquid crystal display devices, a background color or a display color is colored by yellow or blue because of an interference color due to retardation which is a difference between a fast axis and a slow axis based on birefringence and optical activity of liquid crystal molecules. In order to cancel this interference color, for the state where a non-selective voltage is applied, liquid crystal cells opposite to each other in their twist direction which does not have any voltage applying means such as electrodes are stacked or birefringent plastic film is stacked on a liquid crystal cell, thereby compensating for the retardation.
FIG. 2A schematically shows the structure of a liquid crystal element when the non-selective voltage is applied. As seen from the figure, the birefringent plastic film (made of birefringent medium 3) is layered between a polarizer 1 and a substrate 5. The retardation in the birefringent medium 3 and that in the liquid crystal cell are substantially equal and opposite in their polarity. The typical value of the retardation is 800 nm or so.
Changes in the transmission when a voltage is applied to a liquid crystal cell according to the prior art are shown in FIG. 3. As seen from the figure, the retardation when a low voltage (non-selective voltage) is applied is compensated for to provide black display color. However, in the low voltage range where the alignment of liquid crystal molecules starts to change, the alignment including the degree of rising or twisting of the molecules is not stable so that the alignment direction is liable to be non-uniform. As a result, reduction in the contrast, uneven display and crosstalk occur, thereby attenuating the display quality.
The crosstalk is a phenomenon in which multiplex driving cannot place the non-display points (non-selective points) in a complete non-display state but places them in the half selective state, thereby reducing the contrast in the entire display image. This is due to that bias voltages are previously applied to all the electrodes for the purpose of enhancing the response speed so that the liquid crystal at the portion at issue responds slightly to the voltage application; thus the transmission is increased. A difference between the transmission in the state with slight response and that in an ideal non-selective state is called the crosstalk.
The crosstalk phenomenon is remarkable in a normally close system in which the black display is provided when the non-selective voltage is applied. The crosstalk phenomenon changes in accordance with the frequency component of a driving voltage applied. Assuming that in the change of the transmission when an AC voltage is applied to a liquid crystal cell, with the saturation transmission of 100%, the voltage providing the transmission of 10% is a threshold voltage, the crosstalk phenomenon occurs with less degree as the threshold voltage provides less change due to the frequency.
On the other hand, assuming that the twist angle of the nematic crystal is 90.degree. and the product .DELTA.n.multidot.d (.mu.m) of the thickness d of the liquid crystal and the refractive index anisotropy .DELTA.n is 0.5 .mu.m, the TFT liquid crystal display device can provide high contrast of 100:1 or more, multi-gray scale and multi-color display, but provides a greatly changed transmission and display color when it is viewed from its tilted direction (In the field of the TFT liquid crystal display device, .DELTA.n.multidot.d represents the retardation).
In order to obviate such an undesired phenomenon, it was proposed in JP-A-63-115137 that the twist angle in the nematic liquid crystal layer is set for 10.degree.-80.degree. and the product .DELTA.n.multidot.d (.mu.m) of the thickness d of the liquid crystal and the refractive index anisotropy .DELTA.n is set for 0.2-0.7 .mu.m thereby to improve the viewing angle characteristic.
However, the contrast when the display device is viewed from its front was reduced owing to an increase in the black-transmission when the voltage for displaying black is applied. This is attributed to that the liquid crystal molecules in the neighborhood 21 of the substrate surface have not completely risen when the voltage is applied as shown in FIG. 2B. Specifically, if the twist angle is 90.degree., rubbing axes, i.e. alignment directions of the liquid crystal molecules are orthogonal so that the residual retardation is canceled. On the other hand, if the twist angle is 10.degree.-80.degree., the alignment directions are not orthogonal so that the residual retardation is not canceled, thereby increasing the black-transmission.